Thank you for all the comments. I took the watch with me to grab coffee this morning and suddenly things became clear. At first I thought, wow, I must have a bad GPS sensor or something, because this is showing 20MPH higher than my actual speed! And then I realized it's measuring in km/h and I'm a dummy 😏 So I downloaded the highest rated speedometer app I could find for my Android and compared the output to the watch. The results were nearly identical for the entire ride. Also, when outside, the watch only fluctuated between 0.1-0.4 when sitting still. The phone app only shows rounded whole numbers, so I couldn't tell if it has the same fluctuation. I think overall the watch sensor is extremely accurate, as mentioned by Conor. I do wonder if there's any way to calibrate it.. or whether I just do that in my software, asking the user to stand still and checking the low/high values of the sensor during that time.. kind of like calibrating a digital scale.